Microporous film of polyethylene and process for the production thereof

ABSTRACT

Microporous film of polyethylene with a high degree of moisture vapor transmission rate and permeability to air and a process for the production of such a film from a polyolefine by extruding a solution of the polyolefine in a first solvent, followed by cooling, the removal of the solvent and stretching of the film, in which both sides of the film are brought into close contact with a second solvent for the polyolefine before the film is contacted with the cooling agent.

This is a continuation of application Ser. No. 07/836,371, filed on Feb.18, 1992 now abandoned.

The invention relates to a microporous film of polyethylene.

Such a film is known from EP-A-378 279, which describes a process forthe production of microporous films from a solution of ultra highmolecular weight polyethylene in an evaporable solvent.

This film presents the drawback that it is has a limited permeability togases such as air, which limits in particular the use of the film forair filtration purposes.

The aim of the invention is to provide a microporous film ofpolyethylene with an improved permeability to air relative to that ofthe known film.

This aim is achieved according to the invention because the film has apermeability to air of less than 10 s/50 ml and a moisture vapourtransmission rate of at least 7500 g/24 h.m².

The film according to the invention has a very great capability oftransmitting gases such as air, already at a small difference inpressure across the film. When the film is used as a filtering medium ahigh yield of purified gas per time unit can be obtained. Thepermeability to air of the film according to the invention is betterthan 10 s/50 ml, preferably better than 7 s/50 ml. For a goodunderstanding it should be noted here that the time that is required totransport a certain amount of air through the film is used as a measureof the permeability to air and that a smaller number of seconds hencecorresponds to a better permeability to air.

A further added advantage of the film according to the invention is itsvery high moisture vapour transmission ratio which is at least 7500g/24h.m², preferably at least 10,000 g/24 h.m².

The resistance of the film to a difference in pressure as occurs infiltration applications depends on the thickness of the film. Filmsaccording to the invention with a thickness of 10 μm already appear tobe strong enough for many applications. Preferably the thickness of thefilm according to the invention is at least 10 μm.

Another advantage of the film according to the invention is its highabrasion resistance and its high degree of resistance to chemicals,which also make the film suitable for filtration in acid and basicmedia.

The film according to the invention is microporous and consistssubstantially of polyethylene. A microporous film consists of anessentially continuous matrix structure containing small pores orchannels. The size of these pores and channels is between 0.001 and 10μm, preferably between 0.01 and 5 μm.

Polyethylene is here understood to be linear polyethylene with fewerthan 1 side chain per 100 carbon atoms, preferably fewer than 1 sidechain per 300 carbon atoms, and a polyethylene of that kind that mayalso contain minor amounts, preferably less than 5 mol. %, of one ormore copolymerised other alkenes, for example propylene, butylene,pentene, hexene, 4-methylpentene, octene, etc. Such polyethylene can beproduced for example with the aid of a Ziegler or a Phillips processusing suitable catalysts under known polymerisation conditions. Thepolyethylene may also contain minor amounts, for example at most 25 wt.%, of one or more other polymers, in particular an alkene-1-polymer,such as polypropylene, polybutylene or a copolymer of propylene with aminor amount of ethylene. The polyethylene may contain the usualadditives, such as stabilizers, colourants, pigments, fillers and thelike. The weight average molecular weight of polyethylene is determinedwith the aid of the known methods such as Gel Permeation Chromatographyand Light Diffusion or is calculated from the Intrinsic Viscosity (IV),determined in Decalin at 135° C. A weight average molecular weight of,for example, 0.5×106 g/mol corresponds to an IV, determined in Decalinat 135° C., of 5.1 dl/g according to the empirical equation

    M.sub.w =5.37×10.sup.4 [IV].sup.1.37.

Because polyethylene with a high or a very high molecular weight hasspecial properties, such as a high abrasion resistance and a highresistance to many chemicals, use is preferably made of ultra highmolecular weight polyethylene, UHMWPE. Such UHMWPE has a molecularweight of at least 5×105 g/mol, preferably at least 10₆ g/mol.

The invention also relates to a process for the production of amicroporous film from a polyolefine by forming a solution thereof in anevaporable, first solvent into a film, passing the film through a bathcontaining a cooling agent and evaporating the solvent from the film ata temperature below the dissolution temperature and stretching the filmin one or more directions in the plane of the film, wherein the film canhave a thickness upwardly from about 10 micrometers. The film can be140, 150 or 210 micrometers thick.

Such a process is also known from EP-A-378 279 for the production ofmicroporous films, in which UHMWPE is used as the polyolefine, whereinExamples 11, 12 and 13 therein respectively report a film having athickness of 210 micrometers, 150 micrometers, and 140 micrometers.

A drawback of this known process is that the films to be producedtherewith appear to have a moderate permeability to air.

The aim of the invention is to provide a process for the production ofmicroporous films from a polyolefine with a very high permeability toair.

This aim is achieved according to the invention because both sides ofthe film are brought into close contact with a second solvent,preferably being a solvent for the polyolefine, before the film iscontacted with the cooling agent.

With this process it appears to be possible to produce microporous filmswith a very high permeability to air of better than 10 s/50 ml. A largeproportion, usually of at least 50%, and when use is made of UHMWPE aneven greater proportion, of the solution from which the film is formedand hence also of the film formed from the solution consists of thefirst solvent. The same large proportion of the surface consists of thissolvent and it is therefore very surprising that a close contact of thissurface with a second solvent has such a great effect.

A further advantage of the process according to the invention is thatthe film obtained therewith has a very high moisture vapour transmissionrate, which makes the film suitable for use as, for example, a breathingsandwich layer in clothing.

As evaporable solvents use is made of the known polyolefine solventssuch as aliphatic, cycloaliphatic and aromatic hydrocarbons, for exampletoluene, xylene, Tetralin, Decalin, C₆ -C₁₂ -alkanes or petroleumfractions, but also halogenated hydrocarbons, for exampletrichlorobenzene and other known solvents. In connection with theremoval of the solvent, use is preferably made of solvents whose boilingpoints at atmospheric pressure are lower than 210° C., which is the casewith virtually all of the aforementioned solvents.

As polyolefine use is preferably made of polyethylene or polypropyleneor copolymers thereof with at most 5 mol. % of one or more otheralkenes. Polyethylene is preferred on account of its greater resistanceto many chemicals and its great abrasion resistance. Because inparticular polyethylene with a high or a very high molecular weight hasthese properties, use is preferably made of ultra high molecular weightpolyethylene. Such UHMWPE has a molecular weight of at least 5×10⁵g/mol, preferably of at least 10⁶ g/mol.

Homogeneous solutions should be used to produce films from solutions ofa polyolefine. The known methods, for example employing an extruder, canbe used for the continuous production of homogeneous solutions of apolyolefine. Using this method presents the advantage that the solutioncan be prepared and extruded into a film or processed into a film in adifferent manner in one continuous process. The invention is not limitedto such a process though and to a person skilled in the art it will ofcourse be clear that homogeneous solutions prepared in a differentmanner can also be processed into microporous films.

The concentration of the polyolefine in the solution may vary withinwide limits and will usually be chosen between 2 and 50 wt. %, forpractical reasons mainly. Solutions containing less than about 2 wt. %polyolefine usually result in films that are so fragile that it isextremely difficult to process them further. On the other hand,solutions containing more than 30 wt. %, in the case of UHMWPE, and, inother cases, in particular more than 50 wt. %, become increasinglydifficult to process. Concentrated solutions with polyolefineconcentrations of 50 wt. % or more are therefore not preferable althoughit is possible to use such solutions and the use thereof is hence withinthe scope of the present invention. If a portion of the polyolefine iscross-linked before it is dissolved, the processability of the solutionappears to be better in a few cases than if the solution contains thesame total concentration of only non-cross-linked polyolefine. Thisapplies to UHMWPE in particular.

The polyolefine solution is converted into a film that consists of thesolution. This can be done in different manners, for example by spinningit via a spinneret with a very wide slit-shaped nozzle, by extruding itor by pouring it onto a roll or a band.

After a polyolefine solution has been processed into a film the filmconsisting of the solution is passed through a cooling bath containingcooling agent. Preferably, use is made of a cooling agent in which thepolyolefine is not soluble. Water is a very suitable cooling agent. Thetemperature is reduced to such an extent in the cooling process thatgelling takes place in the film so that a structure is formed that issufficiently strong and stable for further processing. It is possible tocool to ambient or an even lower temperature but since the first solventis to be evaporated from the film in the next process step it will beclear that, for a profitable process, it is very desirable to keep thetemperature as high as possible in general. The amount of heat requiredto remove the solvent from the film is thus limited as much as possible.

The first solvent is removed from the film at a temperature below thedissolution temperature, preferably by evaporation but extraction isalso possible. The dissolution temperature is the temperature abovewhich the polyolefine in question can be homogeneously dissolved in thefirst solvent. If this solution is cooled to below the dissolutiontemperature gelling will take place. The dissolution temperature and thegelling temperature may differ from one another to a limited extent. Inthat case, according to the present invention, the first solvent isevaporated from the film at a temperature below the lowest of thosetemperatures.

If the forming method employed permits it, the film may be prestretchedif so desired; this means that the linear rate at which the gelled filmis drawn from the bath or transported differs from the linear rate atwhich the film is formed from the solution. When use is made of, forexample, extrusion the latter rate is the linear rate at which thesolution emerges from the die opening. In the scope of the presentpatent prestretching is defined as the quotient of the rate at which thefilm is transported or drawn from the bath, as described above, and theaforementioned rate at which the solution emerges from the die opening.

In the evaporation of the first solvent the film shows a tendency toshrink. In order to obtain a microporous film this shrinkage must beprevented in at least one direction lying in the plane of the film. Thiscan be done in a simple manner by clamping the film. If the film isclamped in two directions, its thickness is the only dimension that can,and in fact does, decrease. Something similar holds for, for example,tubular film and hollow filaments. It is not only possible to preventshrinkage but even to stretch the film in one or two directions alreadyduring the evaporation of the solvent.

It is also possible to stretch the film in one or more directions afterthe evaporation of the first solvent from the film. This stretching ofthe film from which the solvent has been removed may optionally becarried out at a higher temperature than that at which the stretchingduring the evaporation of the solvent was effected, provided that thishigher temperature is not so much higher than the melting temperature ofthe polyolefine that melt fracture occurs.

In the process according to the invention the surfaces of both sides ofthe film consisting of the solution are brought into close contact witha second solvent before the film is cooled to a gel film through contactwith the cooling agent in the cooling bath. Although the contact of onlya portion of the surface of each of the two sides with the secondsolvent already causes the moisture vapour transmission rate toincrease, it is preferable, for the purpose of simplifying the processand obtaining a film with uniform properties, to bring the entiresurfaces of both sides into close contact with the second solvent.

The close contact can be effected for example by spraying both sides ofthe film with the second solvent in the form of a vapour, a spray ordroplets. Excellent results are obtained when a layer of the secondsolvent floats on the actual cooling agent in the cooling bath. When thefilm is introduced into the cooling bath it then first passes the layerof second solvent, with which it comes into close contact before cominginto contact with the cooling agent beneath the solvent layer.Preferably, use is therefore made of this embodiment of the processaccording to the invention, for example under the conditions describedbelow.

The density of the second solvent must in this case be smaller than thatof the cooling agent. When water is used as a cooling agent thisrequirement is met by most polyolefine solvents.

The thickness of the layer of second solvent is to be chosen such thatthe solvent forms a closed layer and does not form globules on thesurface of the cooling agent. This requirement is usually met if thelayer of solvent is a few, for example 2, millimeters thick. For aperson skilled in the art it is easy to experimentally determine theminimum thickness for obtaining a closed layer for any combination ofcooling agent and second solvent. In order to prevent the risk of thelayer breaking a layer thickness of at least 3 mm is preferred.Preferably, an evaporable solvent is used as second solvent too. Thispresents the advantage that it can be removed together with the firstsolvent already in the film in one and the same evaporation step. For aneconomic process the second solvent is most preferably the same as thefirst solvent.

The layer of second solvent is applied to the surface of the coolingbath in such a manner that both sides of the film are brought into closecontact with this solvent when the film is introduced into the coolingbath. For example, it is possible to place sufficient screen walls atsuitable places in the cooling bath, which screen walls project belowand above the surface of the cooling agent and are set perpendicular tothis surface. In this way it is also possible to considerably limit thearea of the surface of the cooling bath on which there is a layer ofsolvent. A suitable geometry of such screen walls can also ensure thatonly a part of the surface of each of the two sides of the film comesinto contact with the second solvent.

The invention is illustrated with the aid of the following exampleswithout, however, being limited hereto. The quantities mentioned in theexamples were determined in the following manners.

The tensile strength, the elongation at break and the modulus ofelasticity were determined according to ASTM standard D882-83, using aspecimen with a width of 5 mm and a length between the jaws of 25 mm.The crosshead speed was 25 mm/min.

The moisture vapour transmission rate was determined as the MoistureVapour Transmission Rate (MVTR), in g/24 h.m², according to ASTMstandard E96-66BW, at a temperature of 23° C. 50% relative humidity andan air flow rate of 2 m/s

The permeability to air was determined in s/50 ml as the Gurley numberaccording to ASTM standard D726-58, using a measuring area of 6.45 cm²(1 square inch) and a weight of 567 grams. The thickness of the filmswas measured with the aid of a Millitron Feinpruf meter, whose sensorhad a rounding off radius of 12 mm.

The density of the film was determined by weighing a piece of film witha known volume.

The porosity was determined from the measured density ρ and the densityof the polyolefine bulk material ρ₀ as: ##EQU1## The maximum pore sizewas determined with the aid of a Coulter porometer. The IntrinsicViscosity was determined in Decalin at 135° C.

The waterproofness of the film was determined as the height in meters ofthe water column that could be placed on the film before water startedto drip through the film under the influence of the weight of the watercolumn. The loaded surface area measured 17.3 cm². The film wassupported by a metal grid.

EXAMPLE I

A 20 wt. % solution of polyethylene with an Intrinsic Viscosity of 15.5dl/g, which corresponds to a weight average molecular weight of 2.2×10⁶g/mole, in Decalin was extruded at a temperature of 180° C. The extruderhead was fitted with a die with a gap of 400 mm×1 mm. The extruded filmwas introduced into a cooling bath whose liquid surface was about 1 mmbelow the outlet of the die gap. The cooling bath contained water of 20°C., onto which a 3-4-mm thick layer of Decalin had been applied so thatboth sides of the film came into contact with the Decalin as the filmwas introduced into the cooling bath. The solvent was removed from thegel film thus obtained in an oven at a temperature of 70° C., in whichthe length and width of the film were kept constant. The film from whichsolvent had been removed was simultaneously stretched in machine (M) andtransverse (T) direction, at a temperature of 120° C.

Table 1 shows the properties of the stretched film at different stretchratios.

                  TABLE 1                                                         ______________________________________                                        Stretch ratio [M × T]                                                                       6 × 6                                                                            8 × 8                                      Thickness (μm)   55       38                                               Porosity (%)        84       86                                               Max. pore size (μm)                                                                            0.14     0.19                                             MVTR (g/24 h.m.sup.2)                                                                             11200    14640                                            Gurley number (s/50 ml)                                                                           1.5      2.1                                              Tensile strength [M/T] (MPa)                                                                      27/26    27/27                                            Modulus [M/T] (MPa) 330/230  330/260                                          Elongation at break [M/T] (%)                                                                     18/29    15/18                                            ______________________________________                                    

EXAMPLE II

The procedure described in Example I was repeated, with the differencethat only the length of the film was kept constant during theevaporation of the solvent, in this case at 30° C. Next, the film wasstretched at a temperature of 120° C., first in machine direction andthen in transverse direction. No shrinkage was allowed in the directionin which the film was not being stretched.

Table 2 shows the properties of the stretched film at different stretchratios.

                  TABLE 2                                                         ______________________________________                                        Stretch ratio (M × T)                                                                     5 × 10                                                                           5 × 9                                                                            5 × 7                               Thickness (μm) 44       57       81                                        Porosity (%)      84       84       84                                        Max. pore size (μm)                                                                          2.7      3.1      4.3                                       MVTR (g/24 h.m.sup.2)                                                                           13190    12800    12250                                     Gurley number (s/50 ml)                                                                         3.5      1.6      2.4                                       Watertightness (mH.sub.2 O)                                                                     15       7        10                                        Tensile strength [M/T] (MPa)                                                                    13/28     7/25     7/22                                     Modulus [M/T]     170/290  90/280   80/230                                    Elongation at break [M/T] (%)                                                                   25/14    34/13    33/13                                     ______________________________________                                    

EXAMPLE III

The procedure described in example II was repeated, with the differencethat use was made of a 30 wt. % solution in Decalin of high-densitypolyethylene with an Intrinsic Viscosity of 4 dl/g, which corresponds toa weight average molecular weight of about 360,000. Table 3 shows theproperties of the stretched film at different stretch ratios.

                  TABLE 3                                                         ______________________________________                                        Stretch ratio [M × T]                                                                      5 × 5                                                                            7 × 7                                       Thickness (μm)  115      48                                                Porosity (%)       85       81                                                MVTR (g/24 h.m.sup.2)                                                                            11730    13200                                             Gurley number (s/50 ml)                                                                          6.0      4.8                                               ______________________________________                                    

EXAMPLE IV

The procedure described in example III was repeated, with the differencethat the layer of Decalin on the cooling bath was replaced by a 5-mmthick layer of xylene. Table 4 shows the properties of the stretchedfilm at different stretch ratios.

                  TABLE 4                                                         ______________________________________                                        Stretch ratio [M × T]                                                                      6 × 5                                                                            7 × 7                                       Thickness (μm)  92       50                                                Porosity           81       84                                                MVTR (g/24 h.m.sup.2)                                                                            12630    13670                                             Gurley number (s/50 ml)                                                                          4.9      3.2                                               ______________________________________                                    

Comparative Example A

Example II was repeated, with the difference that no layer of solventwas applied to the cooling bath so that both sides of the extruded filmcame into direct contact with the water. The solvent was removed fromthe film, which was then stretched successively in machine andtransverse direction. Table 5 shows the properties of the film.

                  TABLE 5                                                         ______________________________________                                        Stretch ratio [M × T]                                                                      4.5 × 8                                                                          5 × 9                                       Thickness (μm)  69       48                                                Porosity (%)       76       80                                                MVTR (g/24 h.m.sup.2)                                                                            7250     7440                                              Gurley number (s/50 ml)                                                                          171.2    173.5                                             ______________________________________                                    

Both the moisture vapour transmission rate and the permeability to airof the films obtained in this manner are lower than those of the filmaccording to the invention.

Comparative Example B

Example II was repeated, with the difference that a layer of Decalin wasnow applied to the cooling bath in such a manner that only one side ofthe extruded film came into close contact with the Decalin as the filmwas introduced into the cooling bath. The solvent was removed from thefilm, which was then successively stretched in machine and transversedirection. Table 6 shows the properties of the film.

                  TABLE 6                                                         ______________________________________                                        Stretch ratio [M × T]                                                                     7 × 8                                                                            6 × 9                                                                            5.5 × 5.5                           Thickness (μm) 27       37       62                                        Porosity (%)      80       84       83                                        Max. pore size (μm)                                                                          0.13     0.14     0.21                                      MVTR (g/24 h.m.sup.2)                                                                           12300    13990    13090                                     Gurley number (s/50 ml)                                                                         47.5     25.8     24.2                                      Watertightness (mH.sub.2 O)                                                                     >50      >50      >50                                       Tensile strength [M/T] (MPa)                                                                    46/44    35/39    29/19                                     Modulus [M/T] (MPa)                                                                             570/510  400/430  290/250                                   Elongation at break [M/T] (%)                                                                   16/22    19/22    18/27                                     ______________________________________                                    

Although the water vapour transmission ratio is very high, thepermeability to air of the film thus obtained is greater than that ofthe film according to the invention.

EXAMPLE V

Example II was repeated with the difference that the thickness of thelayer of solvent was varied. Table 7 shows the properties of the films.

                  TABLE 7                                                         ______________________________________                                        Thickness decalin layer (mm)                                                                      50       80                                               Stretch ratio [M × T]                                                                       5.5 × 5.5                                                                        5.5 × 5.5                                  Thickness (μm)   61       58                                               Porosity (%)        80       81                                               Pore size (μm)   3-4      3-4                                              MVTR (g/24 h.m.sup.2)                                                                             12500    14500                                            Gurley number (s/50 ml)                                                                           2.4      0.8                                              ______________________________________                                    

We claim:
 1. A microporous polyethylene film having a thickness of atleast about 10 micrometers up to about 210 micrometers, a permeabilityto air of less than 10 s/50 ml, a moisture vapour transmission rate ofat least 7500 g/24 h.m² to 14,500 g/24 h.m² and a pore size between0.001 and 10 μm.
 2. A microporous film according to claim 1 having apermeability to air of less than 7 s/50 ml.
 3. A microporous filmaccording to claim 1, having a moisture vapour transmission rate of atleast 10,000 g/24 h.m² to 14,500 g/24 h.m².
 4. A microporous filmaccording to claim 1, wherein the film has a thickness of up to about115 micrometers.
 5. A microporous film according to claim 1, wherein theweight average molecular weight of the polyethylene is at least 10⁶g/mole.
 6. A microporous film according to claim 1 having a moisturevapor transmission rate of less than 13,990 g/24 h.m².
 7. A microporousfilm according to claim 1, wherein said film has a thickness of up toabout 92 micrometers.
 8. A microporous film according to claim 1,wherein said film has a thickness of up to about 81 micrometers.
 9. Amicroporous polyethylene film having a thickness of about 10 micrometersto about 210 micrometers, a permeability to air of less than 10 s/50 ml,a moisture vapour transmission rate of at least 7500 g/24 h.m² to 14,500g/24 h.m² and a pore size between 0.001 and 10 μm.
 10. A microporousfilm according to claim 9, wherein said film has a thickness of up toabout 150 micrometers.
 11. A microporous film according to claim 9,wherein said film has a thickness of up to about 140 micrometers.